The invention relates to a radiographic image intensifier tube for sensing images formed by penetrating radiation such as X- or .gamma.-radiation. The tube includes an evacuated housing, an input screen for converting an input radiographic image into an electron image, an output screen for detecting an incident electron image, and means for accelerating electrons emitted from the input screen onto the output screen in a focussed manner. The input screen includes supporting substrate, a radiation conversion layer applied to the substrate for converting photons which form an incident radiographic image into photons of lower energy, an electrically conductive barrier layer substantially transparent to said photons of lower energy, and a photocathode layer for emitting electrons into the evacuated space within the housing in response to the incidence of said photons of lower energy.
Such an arrangement in which the barrier layer is a metal layer is disclosed in the German Published Patent Application No. DT 2,321,869.
In known x-ray image intensifiers for example as disclosed in U.S. Pat. No. 3,838,273, the input screen comprises a substrate such as glass or aluminium on which is deposited an x-ray sensitive radiation conversion layer, commonly referred to as a fluorescence layer or scintillator, and formed for example of an alkali halide with an activator, suitably, sodium or thallium activiated cesium iodide. Such a layer usually has a thickness of approximately 300 micrometers and has a granular structure with a rather uneven surface. A transparent barrier layer is applied to this surface before applying a photocathode layer for two reasons. Firstly, in order to provide a more uniform base for the photocathode layer which must be very thin, namely from about 5 to 25 nm because it is related to the escape depth of photoelectrons from the layer. Secondly, to form a chemical barrier between the radiation conversion layer and the photocathode layer, so as to prevent the occurrence of adverse chemical interactions which could reduce the sensitivity of either or both layers and could occur either during manufacture or during the subsequent lifetime of the device, and of course the barrier layer itself must not react in a similarly adverse manner with the other layers. In the above mentioned U.S. patent, a barrier layer is mentioned which is formed by a layer 0.1 to 1.0 micrometer thick of aluminium oxide or silicon dioxide on which is formed a conductive layer 0.5 to 3 micrometers thick of indium oxide to which the photocathode layer is applied, in order to ensure that the whole of the photocathode layer is maintained at a uniform potential during photoemission.
However, with the introduction of larger input screens up to 350 mm in diameter, the conductivity of this form of barrier layer has been found insufficient to maintain the photocathode layer at a uniform potential throughout its surface during higher intensity photographic recording. It has therefore become desirable to employ a thin conductive translucent metal layer such as aluminium as at least part of the chemical barrier, as for example in the aforementioned DT No. 2,321,869, or by allowing a thin layer of aluminium to be formed over an aluminium oxide barrier layer prior to applying the photocathode layer as mentioned in U.S. Pat. No. 3,825,763 and corresponding reissue number U.S. Pat. No. Re. 29,956.
However, in the case of a metal or metal-like conductive layer such as aluminium, a layer which is thick enough to provide an electrically continuous layer over the uneven surface of the radiation conversion layer and to provide sufficient electrical conduction on the one hand while being thin enough to permit sufficient light to pass through, requires to have a thickness of 4 to 10 nm, and this will reflect from about 20 to 50 percent of the incident light from a sodium activated CsI radiation conversion layer whose wavelength is 420 nm (or about 450 nm in the case of thallium activated CsI), and will further absorb about 18% of the light.